What did the immediate ancestor of chimps and humans look like?

Comparing living chimpanzees to living humans, in reference to the species that gave rise to these two closely related species, is one way to frame questions about the evolution of each species.

Generally, it is useful to address evolutionary questions by comparing two living species with the reconstructed "last common ancestor" (LCA) of those species. All of the similarities and differences between the LCA and the living form, in each lineage, represent evolutionary "stories" (that could even be worked out as hypotheses). Similarities indicate important, long-maintained adaptations, and differences indicate evolutionary changes that are ripe for exploration. The different stories that go with each lineage may reflect historically important events, such as the effects of biogeography, climate change, or other changes in the ecology or behavior of the organisms.

One argument that has emerged over the last several years, championed by David Pilbeam and Richard Wrangham, and used by Wrangham and me is our paper on Roots and the evolution of Australopithecus (and related species), is that the chimp-human LCS can be modeled as a chimp-like organism. This argument implies that chimps have not changed much since the chimp-human split, while most of the changes have been along the human lineage. One might think that this is a human-centric approach, but it is not. It is simply taking the available evidence for what it means and going with it.

This argument is based on triangulation. Imagine a phylogeny (family tree) showing gorillas, chimps, bonobos, and humans. Based on the genetic evidence, it would look something like this:

Using apes in general as a reference point, the gorilla-chimp ancestor is likely to have been a chimp-like form. Gorillas seem to have evolved from a chimp like ancestor, with a change in growth pattern to make gorillas both larger and more sexually dimorphic (dimorphic = "different shape") in body size, and to have derived features of their teeth.

Again, using apes in general as a reference point, everything that seems to be different between bonobos and chimps seems most likely to be a derived feature added to bonobos. This suggests that the LCA of chimps and bonobos is more like a chimp than a bonobo.

When we look at fossils of early human ancestors, from back near to the chimp-human split, we see mostly chimp-like features with a mix of derived (added on) features depending on which early hominid species we look at. Most of the differences in postcrania are minor (even given bipedalism) and most of the differences in the skull have to do with a single set of related changes in dentition that relate to a dietary shift. Again, we see chimp-ness.

From the point of view of all of these reference points, the best model is that the chimp-human LCA is most like a chimp, and that gorillas, bonobos, early human ancestors, and of course humans, are all different from chimps in ways that reflect evolutionary novelties.

The chimp, in other words, is the ultimate forest ape, so well adapted to its environment that is has changed very little. All the other species are either close derivations of this form, or more dramatic changes, each of these changes reflecting some environmental (or other) challenge that was, luckily, transformed into an adaptive shift (if you like adaptations), a random change (following relaxed selection?) or extinction. The living forms, obviously, have not yet undergone extinction.